Inductors are used in power converters to store energy in a magnetic field during one part of an operating cycle, and to return all or part of that energy during another part of the cycle. Such inductors are typically comprised of a winding on an easily magnetized or “ferromagnetic” core. One or more so-called “air gaps” in the core are usually required to maximize the energy which can be stored in the inductor. These air gaps may be ‘distributed’ throughout the core, in such materials as “powdered iron” type cores, or may consist of one or more ‘discrete’ air gaps in the core. The faces of a discrete air gap in an inductor are conventionally flat, parallel to each other, and at right angles to the surface of the core outside the air gap.
Various inductor core materials and configurations are known in the art. These materials include silicon-steel (Si-steel) in laminated or tape wound form, ferrite, and amorphous and nanocrystalline alloys (in tape wound form), with benefits and drawbacks to each of these materials in various applications. The present invention applies to tape wound and laminated type inductor cores with one or more intentional discrete air gaps in the magnetic path, and with an alternating current (AC) in a conventional winding (not shown in figures) on the core, and the resultant AC flux in the core.
The distinction between core laminations and tape is largely based on thickness and the method of assembly. Core laminations are relatively thick, typically greater than 0.1 mm, and are stacked or assembled flat. Core tape materials are generally somewhat thinner than 0.1 mm, and are typically wound around a suitable form or mandrel to provide the desired shape.
The energy storage capability of an inductor is influenced significantly by the length of the air gap(s) in its core, there being an optimum air gap length at which the maximum core flux and winding current occur simultaneously, and where energy storage is at a maximum. A “fringe” flux field develops adjacent (but external) to such an air gap, extending from the surfaces of the core on one side of the gap to that of the other side. This fringe field is strongest at the edge of the air gap, and drops off approximately inversely with distance from the air gap.
Referring to FIG. 1A, a perspective view of a conventional inductor core 110 that illustrates this flux fringe field 150 is shown for one surface of the core. A problem associated with an ac flux fringe field is that, as noted in references [1] [2] and [3], at high frequencies and/or flux densities the fringe field 150 induces large eddy currents 117, 118 to flow on the broad surfaces 119, 120 of the tape or laminated core sections 112, 114. These eddy currents induce losses in the core near the air gap, as illustrated by the shaded regions 122 and 132 in FIG. 1b. These losses reduce the ability of the inductor to store and return energy at high frequencies, as the losses are proportional to the square of the induced eddy currents, and thus of both the ac flux density and the frequency. The overall result is a significantly lower allowable maximum power density (rate of energy storage and recovery) for the inductor before overheating occurs. (A similar fringe field enters the core on the edges of the tape or laminations, but this field does not induce excess eddy currents in the core.)
In the related field of Si-steel laminated core transformers, prior art attempts to reduce similar broad core surface eddy currents from the leakage flux field between primary and secondary windings entering the core are known. In this attempt, slots were made in the broad surfaces of the core laminations near the ends of the windings where the leakage flux would enter the core on the broad surface of the laminations. Application of this prior art technique to inductor cores is illustrated in FIG. 2, as taught by the inventor in [4], where slots 287 are cut into the broad surfaces of the laminated core sections 212, 214 near the air gap 226. These slots 287 ‘break up’ the eddy currents, as shown by the eddy currents paths in phantom 227, 228, at the ends of the illustrative flux line 261, reducing their magnitude and the associated losses.
Disadvantageous aspects of this approach include that it is not readily ascertained how long, deep or frequent the slots should be, nor on how to make them. Another disadvantageous aspect is that it is difficult to cut or otherwise form slots in laminated or tape wound material without creating electrical shorts between the cut layers, which increase eddy current losses.
A need thus exists to reduce fringe field induced losses in a tape wound or laminated inductor core and, furthermore, to do so in a manner that is practical, effective, at a reasonable cost and that provides consistent and predictable results.
Ferrite and Nanocrystalline
Ferrite is a well-known inductor core material and has been one of the principal core materials of choice for frequencies above about 5 to 10 kHz due to low hysteressis and eddy current losses. Modern nanocrystalline materials, however, have lower hysteressis losses than ferrites up to about 200 kHz and can operate with 1.6 times the ac flux at 40 kHz and twice the ac flux at 20 kHz for the same loss (based on published data). Furthermore, the nanocrystalline material's saturation flux density BSAT is about 3 times that of ferrites at elevated temperatures of 80-100 degrees C. (1.2 Tesla v. 400 mT). Ferrite, on the other hand, has the advantage of being an isotropic ceramic material, and thus ferrite cores do not exhibit the excess eddy current losses near an air gap experienced by laminated and tape wound metallic core materials.
A need further exists to provide inductors of significantly smaller size, for example, by taking advantage of the properties of nanocrystalline material (or other similar materials yet to be developed) to improve the overall power densities of switching converters, particularly when inductor currents include dc or low frequency ac currents significantly greater than the allowable high frequency ac ripple current.